Conventionally, blending a thermoplastic resin, which per se is electrically insulative, with a conductive filler is a long-known technique for imparting characteristics such as conductivity and antistatic property to the resin, and therefore, a variety of conductive fillers are employed for this purpose.
When a conductive substance is incorporated into resin or rubber which is generally an insulator, for the purpose of imparting conductivity, a phenomenon is observed that the conductivity, which only gradually increases as the filling amount of the conductive substance increases, drastically increases at the point when the filling amount reaches a critical amount and then again gradually increases, so-called “percolation”, a characteristic transition from an insulator to a conductor. It is explained that a three-dimensional network formed in the insulator matrix by an electric conductor causes the phenomenon. The critical amount is called “percolation threshold value” (hereinafter referred to simply as “threshold value”). The threshold value is known to virtually depend on the type of the resin serving as a matrix and the type of the conductor.
Examples of generally employed conductive fillers include carbonaceous materials having graphite structure such as carbon black, graphite, vapor grown carbon fiber (hereinafter abbreviated as “CF”), and carbon fiber; metallic materials such as metallic fiber, metallic powder, and metal foil; metal oxides; and metal-coated inorganic fillers. Among them, in order to attain high conductivity through incorporation of a small amount of conductive filler, use of carbon black or hollow carbon fibrils has been encouraged.
However, when the amount of conductive filler is increased so as to attain high conductivity, melt fluidity of the aforementioned resin composition decreases, leading to difficulty in molding and readily causing short shot. Even when molding is completed, molded products assume poor surface appearance. In addition, unsatisfactory molded products (exhibiting variation in mass per shot or poor mechanical property such as impact strength) may be produced. Thus, in order to enhance conductivity obtained through addition of a small amount of conductive filler, enhancement of conductivity of filler itself has been studied (see, for example, Japanese Patent Application Laid-Open No. 2001-200096).
In an attempt to lower the threshold value, at which conductivity becomes high and stable through incorporation of a small amount of conductive filler by virtue of formation of a conductive network formed by the conductive filler in the conductive resin composition, mainly the following three approaches have been studied.
i) Studies on Effects by Shape of Conductive Filler
The studies have elucidated that the threshold value can be lowered through reduction of dimensions of conductive filler, increase in aspect ratio of the filler or increase in surface area of the filler.
ii) Studies on a Technique of Blending Polymers
With respect to a blended resin having a sea-island structure or a mutually continuous structure in the microscopic configuration, there has been proposed a method for forming a carbon black-matrix resin composite by incorporating carbon black uniformly into the sea phase (i.e., matrix phase or continuous phase) resin compatible with carbon black at high concentration and high density (see, for example, Japanese Patent Application Laid-Open No. 02-201811).
Another method has been proposed for forming a CF-matrix resin composite by incorporating CF uniformly into the sea phase (i.e., matrix phase or continuous phase) resin compatible to CF at high concentration and density (see, for example, Japanese Patent Application Laid-Open No. 01-263156).
iii) An Approach in which the Threshold Value is Lowered by Elevating Interfacial Energy
It has been elucidated that in a composite composition of any of various resins and carbon black, the larger the interfacial energy, the smaller the threshold value (e.g., in a case of polypropylene/carbon black where the interfacial energy is higher than a case of nylon/carbon black, the threshold value is lower). When carbon black is employed as a conductive filler, there has been made an attempt to elevate interfacial energy between carbon black and resin by elevating surface energy of carbon black through oxidation treatment.
The aforementioned studies have been extensively carried out, to thereby steadily lower the threshold value through elevation of conductivity of conductive filler, by means of the polymer blending method, and other means. However, the polymer blending method cannot be applied in the case where a change in intrinsic properties of a starting material caused by blending of polymers is not acceptable. When shape of conductive filler is fined or the aspect ratio or the surface area of the filler is increased, fluidity of the resin composition during molding is impaired. The effect of the method for lowering the threshold value by elevating the interfacial energy is not very remarkable. In this way, there still remain problems such as deterioration of physical properties, lowering of fluidity during molding, poor appearance of molded products, in attaining high conductivity of a resin composition including a single resin system.
Specifically, the commercial demand for reducing adhesion of dust on electric/electronic components to the minimum has been increasing and more and more intensive year by year, along with the progress on technology for downsizing, integration and precision in office automation (OA) apparatus and electronic apparatus.
For example, such a demand is even more prominent there in the fields of IC chips used in semiconductor elements, wafers, interior parts employed in computer hard disks, etc., and adhesion of dust on these parts must be completely prevented by imparting antistatic properties to the parts. For such applications, there has been employed, as a conductive resin composition, a polymer alloy predominantly containing polycarbonate resin (blend of polycarbonate resin with ABS resin) or a polymer alloy predominantly containing polyphenylene ether resin (blend of polyphenylene ether resin with polystyrene resin), into which a conductive filler such as carbon black is incorporated. In order to attain high conductivity, a large amount of carbon black must be incorporated into a resin, resulting in a problem that the mechanical strength and fluidity of conductive resin are lowered.
With respect to automobile outer parts, “electrostatic coating” is applied where a coating having an opposite charge added thereto is sprayed to a conductivity-imparted resin molded product while electrifying the molded product. In the electrostatic coating method, adhesion of the coating onto the surface of molded products is enhanced on the basis of attractive force between the charge on the surface and the opposite charge in the coating. Many exterior panels and parts of automobiles are formed of a polycarbonate resin-polyester resin blend or a polyphenylene ether-polyamide resin blend. When a conductive filler is incorporated into these molding resin materials for imparting conductivity, mechanical strength and fluidity thereof problematically decrease.